Chip-on-board (COB) is a form of integrated circuit (IC) well known in the art. A COB is generally an IC chip attached with an adhesive to a substrate, such as a circuit board, with wires electrically interconnecting the chip to conductors on the substrate. The wires and the interconnection are typically formed using wire bonding techniques, in which very thin electrically-conductive wires, often on the order of about 0.25 millimeter or less in diameter, are bonded to leads on the chip and to bond pads on the substrate. Suitable wire bonds can be achieved with various wire bonding techniques, including thermosonic and ultrasonic bonding.
It is well known to encapsulate a COB die and its wires with an encapsulation material that immobilizes the wires to prevent shorting between adjacent wires and to generally form a protective shell over and around the COB. Encapsulation materials for COBs and other IC devices are often one or two-part epoxies containing a glass filler to reduce the CTE of the material to a value closer to that of the die and circuit board. Reducing the CTE of the encapsulant in this manner reduces the adverse impact that the encapsulant has on the thermal cycle fatigue life of the wire bonds made between each wire and the circuit board and each wire and its bond pad which, during temperature excursions, are subject to thermal stresses as a result of differences in coefficients of thermal expansion (CTE) of the various materials that form the chip, substrate and encapsulant. These thermal stresses can fatigue and fracture the wires at the bond interface, particularly if the assembly is subject to many temperature excursions, high temperatures on the order of 125.degree. C. or more, or intense vibration, all of which are often encountered in automotive and aerospace electronic applications.
Because gaps on the order of about 0.15 millimeter or less are typical between the wires and chip, there is a tendency for non-uniform flow of the encapsulation material, causing void formation as a result of air being trapped within the material. If a void is present, the thermal cycle fatigue life of the COB can be significantly limited due to the void creating a region of high stress concentration. A notable failure mode is for the encapsulation material to delaminate from the chip due to the presence of a void adjacent the chip.
Various deposition techniques have been employed to deposit encapsulants for COBs, some of which are illustrated in FIGS. 2 through 4, which illustrate an inward spiral (FIG. 2), outward spiral (FIG. 3) and serpentine (FIG. 4) paths 110 along which an encapsulation material has been deposited to encapsulate a COB chip 112. However, these techniques have not eliminated the formation of voids in the encapsulant, even when various deposition parameters have been modified, including flow rate, line speed, volume, material temperature, needle size, and distance between deposition needle and substrate. A deposition technique that has been used to reduce void formation entails the use a two-part encapsulant. One of the encapsulant materials is formulated to be more viscous, and then deposited to form a peripheral dam around the COB chip and its bond wires, while the second material is formulated to be thinner, and then used to penetrate and fill the gaps between the wires and chip. A drawback to this approach is that the technique complicates processing and increases costs.
As a result of the above, the use of COBs has been largely limited to consumer electronic applications (whose temperature extremes for thermal cycle fatigue testing are typically 0.degree. C. and 80.degree. C.), and not to more demanding automotive and aerospace applications (whose temperature extremes for thermal cycle fatigue testing are typically -40.degree. C. and +125.degree. C.). However, it would be advantageous if COBs could be employed in the automotive and aerospace industries. The processing of such COBs would preferably be amenable to in-line processes, and employ an encapsulation material that does not complicate the assembly process.